Control system for a variable pitch propeller and its driving turbines



E. CONTROL SYSTEM FOR A VARIABLE PITCH PROPELLER Sept. 9, 1958 M. IRWINET AL AND IIs DRIVING 'IURBINES 9 sheets-sheet 1 Filed Nov. 8, 1950Sept. 9, V1958 E. M. IRWIN x-:TAL 2,851,113 CONTROL SYSTEM FOR AVARIABLE PITCH PRQPELLER AND ITS. DRIVING TURBINES Filed Nov. 8, 1950 9Sheets-Sheet 2 Cttornegs Sept. 9, 1958 E. M. IRWIN ET'AL 2,851,113

CONTROL- SYSTEM FOR A VARIABLE FITCH PRPELLER f AND ITS DRIVING TURBINESFiled Nov. 8, 1950 9 Sheets-Sheet 3 'lrwentors S M L Am T EN Nm .II Ww.mm Mm .I ED N A sept. 9, 19584 CONTROL SYSTEM FOR A VARIABLE FITCHPROPELLER 9 Sheets-Sheet 4 Filed Nov. 8, 1950 E. M. IRWIN ETAL 2,851,113CONTROL SYSTEM FOR A VARIABLE FITCH PROPELLER AND ITS DRIVING TURBINESFiled Nov. 8, 1950 9 Sheets-Sheet 5 Sept. 9, 1958 .III III Il III IIIIIIIIIJ 9 Sheets-Sheet 6 E. M. IRWIN ET AL CONTROL SYSTEM FOR AVARIABLE PITCH PROPELLER AND ITS DRIVING TURBINES Sept. 9, 1958 I FiledNov. 8, 195o Sept. 9, 1958 E,

CONTROL SYSTEM Fo AND ITs DRIVING TURBINES Filed Nov. 8. 1950 M. IRWINET AL R A VARIABLE FITCH PROPELLER 9 Sheets-Sheet 7 Sept. 9, 1958 E. M.IRWIN ET AL CONTROL SYSTEM RoR A VARIABLE PIICH RROPELLER AND IIsDRIVING TuRBINRs 9 Sheets-Sheet 8 Filed Nov. 8, 1950 /DLE i y GttornegsWMM sept. 9, 1958 Filed Nov. 8, 1950 E. M. IRWIN ETAL CONTROL SYSTEMFORA VARIABLE PITCH PROPELLER AND ITS vDRIVING TURBINES 9 sheets-sheet 9United States Patent C CONTROL SYSTEM FR A VARIABLE PTCH PRO PELLER ANDITS DRTVING TURBINES Edmund M. Irwin, Floyd J. Boyer, Arthur W. Gauhatz,and Robert I. Wente, lndianapoiis, Ind., assignors to General MotorsCorporation, Detroit, Mich., a corporation of Delaware ApplicationNovember 8, 1950, Serial No. 194,716

66 Claims. (Cl. Uli-135.72)

This invention relates to control systems for power plants and, moreparticularly, to a control system for an aircraft power plant in which apropeller is driven by two gas turbine engines.

The control system, in its preferred embodiment, is particularly adaptedto the control of a particular propulsion unit embodying a variablepitch governing propeller which may be feathered and which may lbeoperated in direct blade angle control in both forward and reversepitch. The propulsion unit lcomprises clutches by which either or bothpower units may be utilized to drive the propeller. It comprises,moreover, a starting system, fuel supply apparatus, fuel feedcontrolling means,

fuel ignition, and other engine auxiliaries.

A primary object of the invention is to assure and facilitate operationof the power plant most safely and eiiiciently, and with a wide choiceof operating conditions. Another primary object of the invention is toprovide a system by which the various components and accessories of theengine may be organized into a unitary system and the ultimate controlby the pilot or flight engineer of an aircraft made as simple aspossible. Another important purpose of the control system is to providea maximum of llexibility of operation of the power plant with a minimumof controls requiring attention from the pilot. Another important objectof the invention is to provide what may be termed a supervisory controlto insure that conicting and harmful control operations will not bepossible; in other words, to provide proper scheduling and interlock ofvarious functions relating to the power plant.

A further object of the invention is to provide for harmonious operationof the two power units of the engine and of one or both units with thepropeller' under various conditions.

Further and more specific objects of the invention are to provide acontrol which cuts out the power unit if the propeller is feathered; toprovide a control adaptable to various propeliers of the most advancedtypes; to facilitate starting `of the units; to provide for safe,efcient, and automatic control of power clutches; to interloci: :thepower control levers of the two units to prevent improper operation. ofeach relative to the operation of the other and the operating conditionof the power plant; and to coordinate the power control levers of theunit for joint control of both power units and the propeller toeliminate conflicts.

Many other objects and advantages of the invention will be apparent tothose skilled in the art from the subsequent detailed description `ofthe preferred embodiment of the invention, The importance of these willbe apparent to those cognizant of the demands upon and responsibilities`of aircraft iiight personnel, which make it extremely important thatthe operation of the power plant of the aircraft provide for variousprocedures and con-- tingencies, that it be accomplished by thesimpliest possible controls, and that safeguards be provided againstimproper operation. t

" ice Referring to the drawings, Figure l is a schematic diagram of adual power unit gas turbine propeller aircraft propulsion plantincorporating the control system ofthe invention; Figure 2 is a somewhatschematic View of the mechanical structure of a power unit control and apilots control lever assembly; Figure Za is a detail of the yunitcontrol; Figure 3 is a sectional View taken on the plane indicated inFigure 2, further illustrating a power control lever; Figures 4 to 9,inclusive, comprise a circuit diagram of the electrical system of theinvention, which has been subdivided into separate figures according tofunction in view of the complexity of the system and to facilitateexposition thereof; Figure l0 is a diagram illustrating the operatingsequence `of the power control lever and throttle switches; Figures 11and l2 are diagrams of the operating sequences of the limit switchesofthe actuators for the clutches and for the propeller, respectively;and Figure 13 is a chart of a preferred control schedule of anillustrative power plant.

More particularly, with reference to Figures 4 to 9, Figure 4 shows thepower circuits, Figure 5 the starter circuits, Figure 6 the ignition,fuel control, and intake shutter circuits, Figure 7 the clutch circuits,Figure 8 the propeller control circuits, and Figure 9 the interlockingcircuits of the power control levers. It will be understood that thesecircuits are interdependent. However, since the entire system cannot beshown properly on a single sheet, it is believed that clarity ofexposition will best be served by dividing the control circuit betweenythe various figures on a functional basis.

Introduction in view of the complexity of the system, it is believeddesirable to preface the detailed description by a general account ofthe nature of the system and ofthepower plant.

Referring to Figure l, the power plant comprises two identical gasturbine engines A and B, which will be referred to as power units. Sincethese units may be of known type, and since the invention is notrestricted in its application to a particular type of power unit,detailed description of these units is unnecessary. ltmay be pointedout, however, that each gas turbine power unit comprises a turbinedriving a compressor. Atmospheric air taken in through intakes. 1l iscompressed by the compressor, the compressed air is heated by combustiontherein of fuel, and the heated gases drive the turbine` The exhaustgases from the turbine leave the power units through exhaust cones l2providing a rearwardly-directed exhaust which contributes to thepropulsive eifect. Each power unit drives a power loutput shaft 13connected t0 a reduction gear assembly C. Reduction gearing of anysuitable type, indicated schematically by the power unit pinions 16 andthe gear 17, drives a propeller shaft 18 on which is mounted thepropeller D. The power shafts 13 drive lthe pinions 16 through clutches19 which are illustrated schematically in the gure. It will beunderstood that the details of the reduction gear are, in gleneral,immaterial to the invention. The reduction gearing C, and particularly'the clutches 19, are preferably of the type more fully disclosed in thecopending Iapplication of Victor W. Peterson and Herbert H. Schnepel,Serial No. 174,052, tiled July 15, 1950, now U. S. Patent No. 2,838,913.

An external source of power for starting the gas turbine engine isrequired. Preferably, this comprises a starter motor 2l energized from asourcey of compressed air indicated as 22 by an electrically-operatedvalve 23. One starter serves both power units, being coupled by shaft2.4 to a starter selector 26 illustrated more fully in theaforementioned Peterson et al, application by which the starter isclutched to either power unit. The shafts 27 and bevel gears 23 indicateschematically the power transmission from the starter selector to thepower units.

Each power unit has associated with it a number of auxiliary and controlinstrumentalities which are shown in the ligure only for the power unitA. Fuel for the unit is supplied through a line 31 from a source such asa fuel booster pump (not shown) and forced by a pump 32 driven by theunit through a fuel line 33, fuel control 34, line 35, andsolenoidoperated shutoff valve 36 to the fuel burners of the unit (notshown). Details of the fuel control are immaterial to the invention. lngeneral, the functions of the fuel' control are to meter the flow offuel in proper relation to the desired power output from the engine andconditions such as temperature and pressure of the incoming air, thetemperature of the heated gas, and engine speed to provide the desiredpower output without the necessity of detailed attention from the pilot;to promote efficient operation of the power unit; to safeguard the unitagainst overheating or overspeed; to insure the maintenance ofcombustion; to supervise acceleration and deceleration of the powerunit; and to control the engine speed directly by a governor in the fuelcontrol through a part of the operating range.

Such fuel controls are known to those skilled in the art. Basically, thefuel control operates by bypassing a portion of the pump output througha line 37 to the pump input, passing the remainder to the engine. The

amount of fuel allowed to go to the engine is controlled by the factorsand considerations mentioned above. Pressure and temperature of the airat the compressor inlet are transmitted to the fuel control fromappropriative sensitive devices (not shown) through a pressureconnection 38 and a temperature line 39. Pressure may be sensed by atPitot tube and temperature by a fluid-filled thermal bulb, in knownmanner. The utilization of these signals by the fuel control may beaccomplished by mechanisms known to those skilled in the art, thedetails of which are irrelevant to the invention. The control requires apower unit speed input for the speed-responsive mechanism or governortherein, which may be derived in any suitable manner, as by the gearingindicated schematically by bevel gears 40, shaft 41, bevel gears 42, andshaft 43. The fuel control also receives an input indicative oftemperatures in the turbine inlet of the unit from thermocouples 44 andamplifier 46.

The fuel control receives two primary control input signals, lever 47receiving an input indicative of the desired power output of the unitrelative to the maximum power available under the existing conditions,and lever 48 receiving a speed signal to set the governor mechanism ofthe fuel control. These power and speed signals originate in a pilotspower control lever 49 which is coupled mechanically, as by link 51 andarm 52, to a unit control 53. The details of the internal mechanism ofthe fuel control are immaterial to this invention, which may bepracticed with any fuel control having characteristics suited to theengine utilized. A fuel control having both power and speed inputs isshown in British Patent 729,201.

The unit control is an important part of the control system, having forits principal functions the transmission of power and speed signals tothe fuel control and pitch or speed signals to the propeller mechanism.As will be explained more fully, the unit control schedules the signalsto provide for efficient operation of the power unit without thenecessity of calculation by the pilot or flight engineer. Thetransmission from the unit control to the fuel control may be of anysuitable type, preferably a simple mechanical linkage such as push rodsor links 57 and 58 coupled to the crank arms 47 and 48, respectively.

The unit control also supervises other functions of the power unit. Itcontrols the energizatioin of an ignition generator 59 which, through alead 61, energizes spark 4. plugs or other ignition devices by whichcombustion s initiated. The ignition apparatus is preferably of knowntype and need not be described. The amplifier 46 is coupled to theignition apparatus to cut off the ignition when combustion isestablished and re-energizc the ignition if the flame goes out.

The power unit air intake 11 is provided with shutters 62 which areordinarily closed when the unit is out of operation. These shutters areopened and closed through any suitable connection such as the linkageillustrated at 63 by an electro-mechanical actuator 64. This actuator isenergized to open or to close the shutters by a shutter switch 66 by wayof the unit control 53, which prevents closing the shutters when thepower unit is in operation.

The power unit clutch 19 is controlled by a clutch actuator 67 which isindicated for the purpose of illustration in the drawing as operatingthe clutch through a mechanical linkage 68. Preferably, in practice, theclutch actuator is an electrical actuator operating valve by whichhydraulic fluid for engaging and disengaging the clutch is supplied to adouble-acting hydraulic motor and a valve by which cooling fluid issupplied to the clutch during the period of engagement. The preferredclutch and the valve mechanism are disclosed in the abovementionedPeterson and Schnepel application, and the disclosure need not beduplicated here. The clutch actuator comprises an electric motorenergized from the unit control 53. The operation of the clutches isinterlocked so that the clutches may not be brought into engagementsimultaneously and so that control of clutch engagement and cooling isdependent upon the speed of the driving and driven shafts. The speedcontrol is effected in part by speed switches 69, preferably centrifugalswitches, driven by the power unit, which transmit speed signals to theunit control.

Such speed switches are readily available articles of commerce. Examplesof centrifugal switches are illustrated in U. S. Patents Nos. 2,452,298,2,457,192 and 2,621,267. The specific structure of the speed switch isentirely immaterial to our invention.

The fuel pumping mechanism indicated schematically by the pump 32 ispreferably in practice a system including a normal and an emergency fuelpump which may be, for example, of the type disclosed in an applicationof Floyd J. Boyer, Serial No. 139,216, tiled January 18, 1950, for FuelSystems (Patent 2,640,423). The system involves means for bypassing themain pump, to check the functioning of the auxiliary pump, by a checkswitch 71 operable by the pilot.

The auxiliary and control apparatus identified by the numerals 31 to 71,inclusive, is duplicated for the power unit B.

Additional control instrumentalities common to both engines are acoordinating control 72, a start switch 73, and a feather switch '74. Aspreviously stated, the unit control serves to coordinate the variouscontrol operations for one power unit. The major function of thecoordinating control is to coordinate the two power units of an enginewith each other and with the propeller, as will'be more fully explained.

The start switch 73 controls the starter air valve 23 by way of thecoordinating control, which prevents operation of the starter underimproper conditions.

As previously stated, the propeller D is of a feathering type.Feathering and unfeathering of the propeller are initiated by thepilot-operated feather switch 74 and the coordinating control whichenergize an electric -motor in a propeller actuator 76. This actuator isthe primary control for the propeller and is coupled to the propellerthrough a mechanical linkage indicated at 77. By means of the propelleractuator, the propeller may be feathered, may be operated as aspeed-governing variable pitch propeller, may be operated in directblade angle control through a range of positive and negative pitches forground operation, and may be set at zero thrust when the propeller load1is picked up by the power umt.

The coordinating control energizes the solenoids S8 to close the fuelshutoff valves 36-and also deenergizes certain of the unit controlcircuits when the propeller is feathered. The propeller actuator alsoreceives a signal from the power control lever 49 of each unit by way ofthe u'nit controls S3 and the coordinating control, which coordinatesthese signals to transmit a single speed or pitch signal to thepropeller actuator.

The control of the power plant also involves a speed switch device 7S,driven from the propeller shaft 18 by suitable gearing, indicatedschematically at 79, which transmits signals to the unit controls 53.

The operation ofthe propeller as a constant speed variable pitchpropeller is directly controlled by an electrical propeller governor S1which transmits signals to a hydraulic mechanism in the propeller, whichincreases and decreases pitch, through an electrical circuit indicatedat 82. This propeller governor receives desired speed signals from thepropeller actuator thro-ugh an electrical circuit 33. lt also receivesan input indicative of the actual speed of the propeller from analternator 84 driven by the propellershaft through suitable gearingindicated at 86.

The power control lever 49 operates through a normal power control rangeand additionally through a reverse thrust range in which the power plantcontrol is modified to secure both positive and negative thrust forground operation. The reverse thrust control requires differentcoordination of the various elements of the mechanism. This also isachieved by the unit control, which is transferred to reverse thrustcontrol by an electrical signal transmitted through a circuit S7 fromthe power control lever.

The unit control actuates a manual fuel shut-off valve located in thefuel regulator when the power control lever is moved to the stopposition.

Power control levers and unit controls A suitable structure of the unitcontrols 53 and the power control levers 49 is indicated in a general orschematic manner in Figures 2, 2a', and 3. lt will be understood thatstructural details of these mechanisms are omitted from the drawings,since such details are unneces sary to an understanding of theprinciples and application of the invention, and may be varied widely.Figure 2 illustrates the arrangement of the two power control levers andone of the unit controls. The power control levers 49 operate in aquadrant 100, each lever operating through aligned slots 101 and 102 inthe quadrant which constitute the normal operating path of the lever.Figure l0 shows the operating positions of the control lever more fully.It may be noted that slot 102 is shortened in Figure l0, the actuallength being preferably about twice that of slot 101. When the lever isat the left hand or zero degree end of slot 101, the power unit isstopped. As the lever is moved through the slot 101, the engine iscontrolled in starting and idling conditions. At fifteen degrees,conditions are correct for starting and idling on the ground, and `atthirty degrees for idling under operating conditions. Detent mechanism(not shown) may be provided to locate the lever in the Stop, Start, andOperational Idle positions. As the lever is progressed through the slot102, the engine is operated in normal power operation with powerincreasing as the lever approaches the right hand end of the slot. Eachlever may be moved through a gate 103 into a slot 104 in which theengine is operated in what is termed the reverse thrust control range,which is intended for ground operation, and in which both forward andreverse propeller thrust are available. .When the lever is in 'the slot101 or the slot 104, the propeller is under direct blade angle control,so that 5 Y the operation in the slots v*101 and 104 is referred to asthe blade angle control range.

On the other hand, when the lever 49 is in the slot 102, which isreferred to as the governing range, a speed signal is transmitted to thepropeller governorso that the propeller pitch is varied to maintain thedesired propeller R. P. M.

Each lever 49 may be mounted for rotation on an axis 106 fixed in abracket 107 and for sliding movement along the axis (or otherwise) forpassage through the gate 103. The lever is coupled to the pull rod orlink 51, the other end of which is coupled to the input arm 52 of theunit control 53 so that rotation of the power control lever rotates ashaft 108 journaled in the casing 109 of the unit control. The anglesindicated in Figure l0 refer to rotation of shaft 108.

The power control levers are mounted side by side so that they may beoperated together conveniently for n'ormal aircraft operation or singlyfor starting or operation on one power unit.

The unit control 53 includes cam mechanism by which properly coordinatedsignals governing the operation of the power unit in both the normal andreverse thrust conditions of operation are transmitted. These signals,as previously indicated, are a power signal through rod 57 to the fuelcontrol, a speed signal through rod 58 to the governor in the fuelcontrol, and an electrical signal (via the coordinating control 72) tothe propeller actuator controlling propeller pitch in the blade anglerange and propeller speed in the governing range. Since the conditionsof operation are entirely different in the reverse thrust range fromthose in the starting and idling portions of the normal control range,the systemis set up for alternative operation so as to transmit properlyvcoordinated signals in either type of control.

These signals are transmitted by cams 111, 112, and 113 rotated by theshaft 108. As illustrated, these are plate or disk cams with grooves inthe faces of the cams engaged by cam followers. The cams are vfixed to asleeve 114 rotatively coupled to the shaft 108 as -by conventionalsplines 116. The sleeve 114 is shiftable axially of the shaft 108 by afork 117 pivoted on the'casing 109 and engaging between flanges V118 onthe sleeve. The sleeve 114 is biased into the axial position shown,which is that for the normal range, by aspring 119 acting on the lever117, and is shifted axially for the reverse thrust range by a solenoid121 which, vas illustrated schematically, pulls on the lever 117 as anarmature. The power control rod S7, which is slidable in the casing, isVforked to provide cam followers 122 vand 123 alternatively engageablein cam slots in both faces of power cam 111. The speed control rod 5S issimilarly forked for engagement with the two cam slots of cam 112. Oneface provides for normal control, the other for control in the reversethrust range. A third cam follower 124 actuated by the cam 113 issuitably guided for reciprocation in the casing 109, as by a guidemember A126 fixed to the casing. The cam follower 124- transmitselectrical signals to the propeller actuator by means of potentiometers127 and 128, the movable contacts of which are shifted by the camfollower 124.

In the preferred control system, the ymovement of cam follower 124 isthe same in both the starting and reverse thrust ranges. This cam hasonly one groove in which the follower engages in both ranges, asillustrated. Alternatively, this cam may be mounted directly onvshaft108 so as not to be shifted by solenoid 121.

The forms of the slots in the controlling cams are calculated to providethe desired operatingcharacteristics, dependent upon the nature of thepower units, the fuel control, and the propeller and its controlmechanism.

The solenoid -121 is energized whenever the corresponding control lever49 is moved through the gate 103 into the reverse thrust slot by aswitch 131 which Dmay be actuated by the power control lever in anysuitable manner. As illustrated in Figure 3, the switch 131 is anormally open switch which may be closed by a bellcrank lever 132pivoted on the fixed structure of the throttle quadrant assembly andurged into switch-closing position by a spring 133. The lever 132 isnormally held from engaging the switch by a bracket 134 extending fromthe throttle lever and of sufficient arcuate extent to remain inengagement with the lever 134 throughout the angular movement of thelever 49. When the throttle lever is shifted through the gate, the lever132 closes the switch, as indicated by the broken lines in Figure 3.With the solenoid 121 thus energized, the cams 111 to 113 are shifteddownwardly, as viewed in Figure 2, so that the followers engage in theslots in the reverse thrust faces of the cams. This shift takes place asthe throttle lever moves through the gate 103; the radius of the camslots is the same on both faces of the cams at the point correspondingto this angular position.

The supervisory control functions of the apparatus are regulated inaccordance with the position of the power control levers under controlof a number of switches closed through various ranges of angularposition of the lever 49. These switches, referred to as throttleswitches, may be operated in any suitable manner by rotation of theshaft 108. As indicated schematically in Figure 2, an appropriate numberof switch-operating cams 136 fixed to the shaft S provide for actuationof throttle switches 137 fixed in any suitable manner to the housing109. These switches may be of a well-known type operated byreciprocation of a plunger which may be actuated by raised or depressedsectors of the peripheral portion of the disks 136. The operating cyclesof these switches are illustrated in Figure 10, the bars correspondingto the closed position of the contacts.

The unit control 53 also houses a throttle block mechanism whichcontrols movement of the throttle lever out of the governing range inwhich it remains in normal fiight conditions. This throttle block maycomprise a disk or sector 138 (Figures 2 and 2a) formed with a dog 138by a solenoid 142. The solenoid 142 is energized in a manner to bedescribed so that the power control may be moved into slots 101 or 104under proper safeguards but may not be so moved improperly. The latch isformed to permit free advance of lever 49 into the governing range.

The switches 137 operate whether the control is in the normal or thereverse thrust range. A switch T12, olerated only when the power controlunit is in the normal range, is actuated by a cam 146 mounted on the camdisk 111. As will be apparent, when cam 111 is shifted downwardly bythesolenoid 121, cam 146 will not engage the operating plunger of switchT12.

lt does not seem practicable to discuss the structure and the details ofoperation of the control system more fully than has been done in advanceof a detailed disclosure of the operating circuits. Details of thestructure and operation will be considered in terms of the variouscircuits as they are described.

General scheduling of the power plant It may be helpful in understandingthe details of the system to examine, in a general way, the schedule orconditions of operation of the power plant for use with which thepreferred embodiment of the control system is intended. Figure 13 is ascheduling diagram of a dual power unit gas turbine propeller engineemploying the control of this invention. It will be understood thatvalues given in this diagram are illustrative; they are to a certainextent a matter of choice for a given power plant, and may vary greatlyfrom one power plant to another. The abscissa of the curves of Figure 13is the control setting, which is the rotation of shaft 108 of the unitcontrol. Scheduled engine R. P. M. and propeller shaft horsepower areindicated by solid lines in percent of the maximum value. The governorsetting, by which is meant the setting of the governor in the fuelcontrol, and the throttle setting, by which is meant the power controlsetting of the fuel control, are indicated by broken lines, likewise inpercent of the maximum setting. Blade angle in the blade angle controlrange is plotted in degrees. As will be seen, the total travel isindicated as 90 degrees, zero degrees being the Stop position (sce alsoFigure l0), the range from 2 degrees to 28 degrees being the blade anglecontrol range, and that from degrees to 90 degrees being the governingrange in which the propeller pitch governor is in operation.

The curves for shaft horsepower and R. P. M. are for normal poweroperation through the reverse thrust and governing ranges, and do notinclude the starting range.

The curves for governor setting and throttle setting are two-valued inthe blade angle control range, the broken lines indicating the settingin the reverse thrust range and the broken crossed lines indicating thevalues when the power control lever is in the slot 104 in which startingis effected.

Perhaps the most fundamental setting isthe throttle setting which isadvanced from zero or complete cutoff in the Stop position to about forstarting the engine, and then increases, first gradually and then moresharply, as the power control is moved to the degree position for takingoff. The Start and Ground Idle throttle settings are set in accordancewith the operating characteristics of the engine to give sufficientpower for consistent operation and to handle the propeller load. Thethrottle setting curve through the governing range is preferably such asto give a substantially straight line curve of shaft horsepower, asindicated. In the reverse thrust range, the throttle setting remainssubstantially constant through the positive thrust portion of the rangebut increases sharply toward the maximum negative thrust position of thecontrol to provide suicient power for braking. The shaft horsepowercurve rises accordingly in the negative pitch range. The effect of thegovernor in the fuel control on operation in and near the blade anglecontrol range will be discussed presently.

As will be noted from the blade angle curve, the blade angle decreasesfrom about l2 degrees at the 28 degree control position to zero at aboutthe l5 degree control position and increases negatively to minus 20degrees pitch at the two-degree position. Throughout this range, bladeangle is determined directly and solely by the position of the powercontrol lever. No curve is given for blade angle in the governing rangesince, in this range, there is no fixed blade angle, the angle beingadjusted by the propeller governor to maintain the desired engine speed.

The engine speed or R. P. M. curve shows only slight variation becauseof the characteristics of the gas turbine` which lis essentially aconstant speed power plant; that is, it does not admit of wide variationin speed like an internal combustion engine. Basically, engine R. P. M.is controlled in the blade angle control range by the governor in thefuel regulator and, throughout the governing range, which is OperationalIdle and above, by the propeller governor.

In a limited range above Operational Idle, the control of engine R. P.M. and propeller shaft horsepower is effected by both governors, theactual operating points varying within the shaded areas bounded by thelines identified as Static and M. P. H. Under static conditions theengine is governed to deliver about 15% rated power. However, assumingthat the plane is landing at a speed of 100 M. P. H. with the control inOperational Idle for minimum operational power, the propeller governorcan reduce the pitch only to a minimum value. The 100 M. P. H. relativewind acting on the propeller generates energy which is transmitted tothe engine, so that at the Operational Idle point and 100 M. P. H. speedthe propeller actually furnishes power to the engine amounting to about2% of maximum engine rating. Above about a 40 degree control setting,the propeller governor takes over, the increased engine power at thissetting being sufiicient to require governing action by the propellergovernor. The area between the engine R. P. M. curves for 100 M. P. H.and static conditions represents the deviation in engine speed from thescheduled speed which occurs at reduced throttle conditions. Maximumdeviation occurs at static conditions, zero deviations at 100 M. P. H.,and proportional deviation at speeds between these two values.

It will be noted that the fuel regulator governor setting is at about90% rated engine speed through the negative pitch portion of the reversethrust range, then decreases to approximately 80% rated speed atOperational Idle. The governor setting increases rapidly with furtheradvance of the throttle so that, in normal operation in the governingrange, the speed governor does not control the engine and does notconflict with the propeller governor. The governor setting at Start isin the neighborhood of 75%.

The blade angle at Start is the value corresponding to minimum torquerequirements of the propeller which, in the particular installationchosen for discussion, is approximately minus 6 degrees as indicated onthe figure. Operating conditions when the engine has started depend uponthe throttle and governor settings, resulting in a speed for the enginewhen it is brought up to speed with the control in Start position ofabout 80% rated speed. The speed differs slightly with the clutchengaged due to the propeller load from that of the engine runningunloaded, as indicated by the width of the shaded Speed (Start) area onthe diagram.

It will be understood that Figure 13 is illustrative of the conditionswith both power units in operation. The control is such that it isadapted to operation on a single power unit. With a single power vunitin operation, the propeller governor, the fuel regulator governor, thethrottle, and the blade angle are set to the ysame values as when twounits are in operation. The horsepower available will, of course, beonly one-half that for two units. Throughout the governing range, theprincipal effect of the reduced horsepower is simply to cause thepropeller governor to reduce the propeller pitch so that the single unitcan drive it at the scheduled speed. In the reverse thrust range, thescheduling is such that one power unit may handle the entire load. Itwill be noted that the maximum braking Vload is considerably under thefull power of one unit. The power or throttle setting is sufiicientthroughout the blade angle control range for one unit to handle thepropeller load. When both units are in operation, as is usually thecase, an excess of power is available. In this case, however, thegovernors in the fuel regulators reduce the fuel supply to theindividual units so that the speed and power of the two units follow theindicated schedule. Any variation between single and double-unitoperation due to governor regulation characteristics is immaterial.

The principles of scheduling involved in the control will `be clear tothose skilled in the art fro-m the above. The absolute values 'of thequantities indicated in Figure 13 and the relative values at differentpoints through the range of the control will vary with the desiredoperating characteristics and with the type of power plant and loadinvolved. The actual cam contoursA will also vary, of course, with thespecific characteristics of the fuel regulator including the governor,the propeller governor, and the linkages or other connections betweenthe cams and these controlling devices. Determination of the actualvalues for a particular installation is simply a matter of engineeringdesign.

Power circuits As a feature of the organization -of the control system,several busses are provided to furnish power to the con- 10 troldevices, the energizationof these busses being related to the operatingsetup at any given time. To avoid unnecessary duplication in the circuitdiagrams, the organization of this bus system or power control circuitis illustrated in Figure 4.

Power for the operation of the control is derived from a D. C. power bus150 in the aicraft. A nacelle bus 151 for each engine or dual power unitis energized from the power bus by a relay 152 energized by amanually-operable switch 153 under. control of the pilot or ightengineer. This bus may energize fuel booster pumps, fuel tank shutoffvalves, etc. (not shown). Alternating current for the thermocoupleamplifiers and propeller governor is generated by an inverter 154connected to the D. C. power bus 150. The output of the inverter is fedto the 400 cycle bus 155 through front contacts of a relay 156 energizeddirectly from the nacelle bus 151.

The additional D. C. busses are identified as 1, 2, 3, and A4, bus 1being energized from the nacelle bus 1951 through a fuse 157 so thatthis bus is constantly energized whenever the power plant is set up foroperation by energization of the nacelle bus.

It may be noted here that, throughout this specification, the variousswitches, relays, and other electrical apparatus common to both powerunits and those individual to the A unit will be indicated by unprimedreference characters, and devices individual to the B power unit bycorresponding primed reference characters. Throttle switches areindicated by T and speed switches by S. Relays in general are identifiedby R, relays in the clutch control circuits by C and relays in thepropeller circuits by P.

Proceeding with 'the description of the power circuits, bus 2 isenergized from bus `1 through the normally open contacts of a powerrelay RP energized by throttle switches T1 and T1- in 4parallel which,as indicated in Figure l0, are closed except when the power controllever is in the'Stop position. Thus, bus 2 is energized when either unitis in operation, but is cut out whenever the power controls forboth-units are moved to the Stop position.

Bus 3 is energized from bus 2 through back contacts RF1 of relay RF, thefeather relay, which is energized whenever the controls are actuated tofeather the propeller. The relay remains energized as long as thepropeller remains feathered and bus 2 is energized.

Bus 4 is energized from bus 3 through the back contacts RR1 and RRl, inseries, of relays RR and RR', the reverse thrust relays of each unit.Relays RR and RR are energized by the power control lever switch 131 ofthe corresponding unit when the lever is shifted through the gate 103into the reverse thrust slot.

In summary, it may be pointed out that bus 2 is energized except whenboth power units are cut out; bus 3 is energized whenever bus 2 isenergized unless the propeller is feathered or the feathering movementhas been initiated; and bus 4 is energized whenever 'bus 3 is energizedexcept when either control lever is shifted into the reverse thrustslot.

Starter circuits The starter circuits shown in Figure 5 are vsolelythose concerned with the operation of the starter selector to connectthe starter to one or the other power unit and with the energization -ofthe starter actuator to supply compressed air to the starter. Thestarter selector 26 comprises a coil 161 for engaging the clutch bywhich the starter is connected to power unit A and a coil 162 forclutching the starter to power unit B. The clutching mechanism isdescribed in the above-mentioned Peterson et al. application; however,an understanding thereof is not necessary for the understanding of thisinvention. Coil 161 is energized by throttle switch T2 and coil 162 bythrottle switch T2', these-switches being closed except when thecorresponding power lever is in Stop position.

The energizing circuits proceed from bus 4 through T2 or T2', line 163or 163', front contacts RS1 or RSZ of the starter relay RS and coil 161or 162 to ground. The starter relay RS, which may be in the coordinatingcontrol 72, is energized from bus 4 through throttle switches T3 and T3(in parallel), speed switches S1 and S1', the pilot-operated startswitch 73, and theRS relay coil to ground. The start button 73 is heldclosed by a solenoid 164 coupled to the movable contact and energized inparallel with relay RS.

Switches T3 and T3 are closed only when the corresponding power lever 49is in Stop position, Switches S1 and S1' are closed below a low butself-sustaining running speed of the units A and B respectively, whichspeed may be taken as 5900 R. P. M. Thus, in order to energize thestarter relay, the power control of the unit being started must beadvanced from Stop, the other power control must be at Stop, thepropeller must be unfeathered (to energize busses 3 and 4), both unitsmust be below 5900 R. P. M., and the pilot must actuate the start switch73. The starter relay closes contacts R51 and RSZ in the circuits to thestarter selector 26, energizing the coil 161 or 162 corresponding to theunit the control of which has been advanced.

Movement of the unit control to Start position closes its throttleswitch T4 or T4' to actuate the starter. These switches are connectedinparallel in a circuit from bus 4 through front contacts RSS of thestarter relay lead 165, switches T4 and T4', lead 166, and valve-openingiield coil 23h and armature 23a of the motor of the air valve actuator23, which opens the air valve and thus supplies power to the startermotor 21. The starter motor cranks the selected power unit, acceleratingthe unit until its speed reaches 5900 R. P. M., at which point the speedswitch S1 opens, deenergizing the starter relay and the solenoid 164,releasing the start button 73. When the starter relay deenergizes, itcloses back contacts RS4 which make a circuit from bus 2 through line167 and the valve-closing iield coil 23e and armature 23a of the airvalve actuator, operating it in the reverse direction to shut off theair supply to the starter. The actuator motor 23 is a well-known type ofreversible motor, and includes limit switches 168 and 169 which operatein the usual manner to open the circuit to the motor when the actuatorhas traveled through its full range, thus deenergizing the openingcircuit when the valve is open and the closing circuit when the valve isclosed.

Since all the starter selector and starter energizing circuits aresupplied from bus 4, they are inoperative if the propeller is featheredor either unit control is in the reverse thrust range.

As will be apparent, the starter air valve circuit could easily bemodified to provide a relay circuit for control of an electric startermotor if such were desired. The significance of the starter circuitswill be more fully apparent after the description of related circuits.Lines 177, 177', 206, and 206', indicated on Figure 5, relate to otherportions of the system, to be described. Switches T2 energize a numberof circuits relating to the corresponding unit when the power control ismoved from Stop. This common energizing circuit may be regarded as aunit bus.

Intake shutter circuits Figure 6 illustrates the ignition, fuel, andshutter circuits. The circuits shown are for one power unit only, beingduplicated for the other unit.

Shu ttcrs 62 (Fig. l) are usually closed except when the pov/er unit isin operation. Particularly when a power unit is shut down in ight, it isdesirable to close the shutters so that ram air will not rotate the idlepower unit. The shutters are opened and closed by the actuator 64, whichis a standard reversing electric motor actuator with limit switches,under control of the shutter switch 66 and throttle switch T5, which isclosed when the control is at Stop.

Switch 66, which is actuated by the pilot or flight engineer, isnormally open and may be closed on one contact to energize the shutteropening field winding 64b and armature 64a of the actuator 64 from thenacelle bus 151, or on the other contact to energize the shutter closingfield 64e and armature 64a through throttle switch T5. These circuitsare made through the usual limit switches actuated by the motor, of thesame nature as those previously described for the air valve actuator.Switch T5 is closed only when the power control of the unit is at Stop,thus providing an interlock to prevent closing of the shutter when thepower unit is in operation.

Fuel system circuits The elements in Figure 6 which are included in thefuel system are enclosed in a broken line. They include the solenoid 83which closes the normally-open fuel shutoff valve 36 (Figure l) whenenergized by a circuit from bus 2 through front contacts RF 2 of thefeather relay` line 170, and solenoid 88 to ground. The fuel line isthus closed when the propeller is feathered, regardless of the positionof the power control lever 49.

The pump checkout solenoid 171 in pump 32 is energized from the nacellebus through the pilot-operated check switch 71 and line 172.

A pump failure warning light 173 indicates incipient failure of the mainor primary fuel pump. This light is actuated by a change in pressurewithin the fuel system, which is all that need be known about it for thepurpose of understanding the present invention; it is more fullydisclosed in the abovementioned Boyer application. The light 173 isenergized by a switch 174 actuated by a fluid pressure responsive device176 in the fuel system. An energizing circuit for the switch 174proceeds from bus 4 through throttle switch T2, which is closed exceptwhen the control lever is at Stop, and lead 177. Thus, the warning lightis energized whenever bus 4 is energized, the power control lever isaway from Stop, and the pressure switch is closed.

Switch T2 also energizes a circuit from bus 4 to the fuel shutoffsolenoid 88 through lead 177, speed switch S2, lead 178, back contactsRF3 of the feather relay, and lead 170. Switch S2, which is one of thespeed switches in the unit 69, is closed below a minimum suitable speedfor initiation of combustion in the power unit, which may be 1700 R. P.M. Thus, when the propeller is unfcathered and the power control ismoved away from Stop so that the unit can be started, the solenoid 8S isenergized to keep the fuel shut oif until the unit is accelerated to1700 R. P. M., at which point the unit will provide sufficient aircirculation for proper ignition of the fuel. This arrangement preventspossible ooding of the unit by supplying fuel at too low a speed, orimproper combustion in the absence of suiicient air. At 1700 R. P. M.the power unit is capable of assisting the starter in bringing the unitup to a speed at which the unit is self-sustaining. Thus, the starter,aided by the turbine in the unit, continues to accelerate the unit until5900 R. P. M. is reached, at which point the unit is more thanself-sustaining and the starter is cut out by switch S1 or S1' (Figure5).

Ignition system control The ignition system preferably includes sparkplugs in the unit (not shown) and an ignition current generator whichsupplies a high potential to the spark plugs. Such systems are wellknown, and the control system of thc invention is applicable to variousignition systems of this and other types. The invention is concernedonly with control of the energization of the ignition generator, whichis identified as 59 in Figures land 6. The control system provides forenergizing the ignition when the starting operation is initiated,de-energizing it when the flame has been established, and automaticallyreenergizing the ignition if a blowout of the flame occurs. The controlincludes the thermocouples 44 which respond to tempera-` ture in theturbine inlet, the output of which is amplified by amplifier 46energized from the 400 cycle kbus 155. The output circuit 181 of theamplifier is energized when the temperature to which the thermocouplesrespond reaches a value well below the operating range but suilicientlyhigh to indicate the presence of combustion; such, for example, as 800degrees F. This output energizes temperature relay RT, the frontcontacts of which close a circuit from line 177 through the coil ofignition relay Rl to ground. The ignition generator 59 is energized fromline 177 through the back contacts of the ignition relay RI and line182.

When starting the unit, the unit is cold, relay RT is deenergized, andthe circuit from line 177 to the ignition system is closed by relay Rl.Therefore, when the power control lever is moved away from Stop, switchT2 is closed, providing ignition in the combustion chambers so that,when the fuel is injected at 1700 R. P. M., it is immediately ignited.The ignition circuit stays energized until the thermocouple amplifierresponds at 800 vdegrees F., closing the contacts of relay RT and break-,ing the ignition circuit at relay Rl. As will be apparent,

if the llame goes out, the ignition circuit will be re-established bythe thermocouple `amplifier and relays RT and RI unless the powercontrol has been returned to the Off position or bus 4 has beendeenergized.

Clutch control circuits The circuits previously described areinterrelated with the circuits by which the clutch actuators 67 arecontrolled, illustrated in Figure 7. These clutch circuits areparticularly adapted for use with the power plant and clutch systemdisclosed in the above-mentioned Peterson et al. application. However,they may, with appropriate modifications, be applied to other clutcharrangements, and it is believed that the principles of the inventionare capable of wide application to various power plants and clutcharrangements therefor.

By way of introduction, it may be repeated that the clutches disclosedin the .Peterson et al. application are engaged and disengaged byhydraulic motors controlled by valves operated by actuators. Theactuator for each clutch also controls a valve to supply oilto theclutch for cooling during the period of slip after engagement. Theactuator has three positions of rest: a clutch disengagement position, aclutch engagement position requiring full travel of the actuator fromthe disengagement position, and a coolant shutoff position involvingmovement part way back to the disengagement position. The supply ofcoolant is initiated by movement to the engagement position andterminated by movement to the coolant shutoff position. The clutch isdisengaged by ycompleting the return movement of the actuator.

The purposes and functions of the clutch control of the invention may begenerally summarized as follows: The system provides for clutching thepropeller automatically to an operating power unit, for clutching thesecond power unit to the power-driven propeller for starting thereof,and for clutching the idle units in sequence to a windmilling propellerfor an air start of the units. The system insures that these operationscan take place only under proper conditions of setting of the powercontrol levers and speed of the power units and propellers. lt alsointerlocks the clutches so that one clutch may not be engaged as long asthe cooling oil is being supplied to a previously engaged clutch,because a clutch should not be required to pick up the load of thepropeller and an idle engine at the same time, and because the supply ofcoolant is not adequate for` two clutches,

These functions are assumed by circuits involving connections to thefeather relay RF and starter relay RS, a number of throttle switches,speed switches responsive to the rotation of each power unit and of thepropeller, and a number of relays in the operating circuit of eachclutch.

14 Referring now to Figure 7, it may be pointed Vout that this figureillustrates the circuits involved in the control of the clutch of the Aunit. Since the control of the clutches is interlocked, this includescertain of the control instrumentalities of the B unit clutch which 'areenclosed in a brokenline rectangle in the'iigure. 1t will be understoodthat the control lsystem Afor the lB unit is identical to that shown forthe A unit and involves the same cross-connections. It-has, therefore,been omitted from i the drawings to avoid unnecessary duplication andcomplication.

We may `begin the description of the system by tracing the circuitsinvolved in engagement of the A unit clutch with that unit in operation.Since the propeller Iis 'unfeathered before starting the unit, contactsRF4 of the feather relay RF will be open. In starting the power unit,the starter relay is energized, closing its 'front contacts R85, RS6,and RS7. As vpreviously explained (Figure 5), the starter relay is heldin until "the lunit reaches 5900 R. P. M., when it is opened'by speedswitch S1. While closed, contacts RSS energize a clutch transfer relayCT through lead 185. Relay CT completes a selfholding circuit from bus 4through lead 186, propeller speed switch S4, line 187, and frontcontacts CTI. Switch S4, which is inthe speed switch unit 78 driven bythe propeller shaft, remains closed until the propeller shaft reaches aspeed somewhat below the normal range for propulsion, which 'may be, forexample, a'speed corresponding to 11,200 R. P. M. of the engine,referred to hereinafter as 11,200 equivalent R. P. M.

Contacts R56 ofthe starter relay close a circuit from bus 4 through line188 to the coil of the ground start relay CG. Relay CG sets up a`self-holding circuit through throttle switch T 6, line 189, and itsfront contact CG1. Switch T6 is closed only in the lStart position 'andthus holds relay CG energized 'until the power control is advanced fromthe Start 'setting Lines 185 'and 188' energize transfer relay CT andground :start relay CG respectively, of the B unit'clutch-control (notshown), and line 187 from speed switch S4 provides the holding circuitfor CT.

Energization of relays CT and CG sets up a circuit for engaging theclutch through speed switch S3, which closes when the power unit reachesa speed slightly below that at which S4 opens, say 11,000.12. Thus, withthe unit control in Start, as the unit becomes selfsustaining andaccelerates to 11,000 R. P. M., at 'which point it is capable of takingon the propeller load, the clutch is engaged. The circuit .is from bus 4through T6, lead 191, front contacts CTZ, switch S3, front contacts CTS,line 192, front contact CGZ, line 193, throttle switch T7 of the B unit(which is closed only when the B unit control is at Stop), line 194,back contacts CC1 of the coolant control relay CC, line 195, closedcontacts 197er, and engaging field winding 67e, brake release coil 67r,and armature 67a of the clutch Valve actuator motor 67 to ground. Switch197 is one of three limit switches 197, 198, and 199 operated by thearmature 67a. Figure 7 shows these switches in their condition when theactuator is in disengaging position. .Figure 11 is a timing diagram ofthe switches. Switch .197 is closed on contact 197a until the actuatorsubstantially completes its travel to the engaged position, when thisswitch is thrown to contact 197b. Movement of the switch 197 completes acircuit from the clutch-engaging line through Contact 197b, line 201,and the coil of pilot switch by-pass relay CB. This relay shuntsthrottle switch T7 at its front contacts CB1. It also completes aself-holding circuit from the engagement line 195 through line 202 andfront contacts CB2. Relay CB is provided to prevent deenergization ofthe clutch-engaging line by moving the power control of the 'other unitfrom Stop. lt makes possible, if desired, the use of a solenoidtypeactuator which must be held energized to continue the flow of coolingfluid to the clutch.

As stated, the energization of engagement line 195 occurs when the unitreaches 11,000 R. P. M. The inertia load and drag of the reduction gearland propeller decelerate the unit slightly, but the dropout point ofswitch S3 is low enough that this switch remains closed. As thepropeller load is accelerated, the clutch synchronizes after an initialperiod of slip and the power unit regains speed until the propeller isturning at a speed equivalent to 11,200 unit R. P. M. At this point,switch S4 opens, breaking the holding circuit of transfer relay CT. Indeenergizing, this relay completes a circuit from bus 4 through lead203, back contacts CT2, switch S3, back contacts CT3, line 204, and thecoil of the coolant contral relay CC to ground. In energizing, relay CCopens back contacts CCI, deenergizing the clutch-engaging line 195 andthereby by-pass relay CB. Relay CC makes a circuit from bus 4 throughthrottle switch T2, which is closed except in Stop, line 206, contactsCC2, line 207, contact 19811 of the limit switch 198, and disengagingeld winding 67d, brake release coil 67r, and armature 67a of the clutchvalve actuator. This circuit energizes the actuator for reverse rotationto terminate the coolant supply. This coolant shutoff circuit is mainntained energized through switch T2 until the actuator 67a has closed thecoolant valve, unless the unit is stopped. When the actuator has rotatedin the reverse direction a sucient distance to cut: olf the coolant,switch 198 closes on contact 198a, opening the circuit to the motor 67.

The circuit just described provides for continuing the supply of coolantuntil the slip period of the clutch is terminated. The engine is now innormal operation with one power unit operating the propeller, and thepower control may be advanced into the governing range or into thereverse thrust range for operation of the aircraft, although normallythe second power unit would be started before any such operation isundertaken.

When the coolant flow is cut olf, a circuit for energizing the B unitclutch is prepared. This circuit is from bus 4 through T2, line 206,contacts CCZ, line 207, contact 198a, line 213, and the coil of the Bunit second clutch engagement relay CS to ground. This circuit, thepurpose of which will be explained, is not activated until the lirstclutch is engaged and the coolant supply has been terminated.

Having been engaged, the clutch may be disengaged either by actuatingthe controls to feather the propeller or by moving the unit control toStop. If the propeller is feathered, the feather relay RF closes acircuit from bus 2 through front contact RF4, line 208, contact T8b ofthrottle switch T8 (closed except in Stop), clutch-dis engagement line209, limit switch 199, disengage field 67d, and armature 67a to ground.The actuator is thus energized to complete its return movement,reversing the clutch engagement valve and disengaging the clutch. Thismovement is terminated by the limit switch 199. It will be apparent thatthis control prevents any attempt to drive a feathered propeller. If theunit control is moved to Stop, switch T8 closes on contact TSa,energizing the disengage line 209 from bus 1 through line 210 andcontact T8a. Thus, either unit is automatically declutched by bringingthe power control of the unit back to Stop.

A circuit is also provided to disengage the clutch when the power unitis started. When the starter relay is energized, contacts RS7 energizeline 212 from bus 4, thus energizing contact TSb. When the unit powercontrol is moved out of Stop position, contacts T8b close, energizingthe clutch-disengaging line 209.

The system also provides for engaging the clutch of one power unit tostart that unit when the other power F unit is already in operation. Themanner in which the clutch is engaged when the unit is operating in theStart and Ground Idle power position has been explained for the A unit.Assuming that the A unit has not been started but that the B unit isoperating in Ground Idle 16 condition,'we'may now trace the operatingcircuit and procedure for starting the A unit. The feather relay RF andthe starter relay RS will be deenergized. Coolant control relay CC isalso deenergized.

When the B unit clutch has completed the engagement cycle, the B unitclutch actuator 67 energizes the second clutch engagement relay CS ofthe A unit through lead 213', as previously described for relay CS'. Forstarting the A unit from the B unit, the power control of the B unit isleft in the Start position. Ground start relay lGG of the A unit, whichwas energized by the starter relay, is held energized from switch T6' ofthe B unit,

' since the holding circuits of relays CG and CG' are encrgized inparallel through leads 188 and 188' and holding contacts CGI and CG1.Also, with the B unit in Start position, switch T7 will be open. As longas the A unit control is in Stop, the disengage circuit of the A unitclutch will be energized from bus 1 through lead 210, contact T8a, andlead 209. This circuit will be broken at T8Q when the A unit powercontrol is moved away from the Stop position. The clutch transfer relayCT, the holding circuit of which is connected in parallel with that ofrelay CT through lines 187 and 187', will have been deenergized byopening the holding circuit at speed switch S4 when the propellerreaches 11,200 equivaient R. P. M.

The second clutch engagement relay CS sets up the circuits by which thesecond clutch to be engaged is freed from control by the unit speedswitch S3 and the throttle switch T7 of the other unit. Contacts CS1bridge leads 191 and 192, thereby shunting the front contacts CTZand'CT3 of the transfer relay. Contacts CS2 connect leads 193 and 194,thus shunting the throttle switch T7'. When the power control of the Aunit is brought to Start position, switch T6 is closed. completing acircuit from bus 4 through lead 191, contacts CS1, lead 192, frontcontacts CG2. lead 193, con tacts CS2, lead 194, contacts CCI., and lead195 to the clutch valve actuator 67, which functions to engage theclutch as previously described. It may be noted that switch T6 alsocloses an additional holding circuit for relay CG through lead 189 andcontacts CGI.

As the A unit is brought up to speed by the operating B unit, fuel isadmitted at 1700 R. P. M. by switch S2 (Figure 6) and the ignition,which was energized by switch T2, is cut out by the ignition relay whenthe temperature reaches S00 degrees F., just as when the unit is crankedby the starter. When power unit A reaches a speed of 11,000 R. P. M.,speed switch S3 closes, energizing a coolant shutoff relay circuit frombus 4 through lead 203, back contacts CTZ, switch S3, back contacts CTS,lead 204, and relay coil CC to ground. Relay CC establishes the coolantshutoff circuit, as before, from bus 4 through switch T2, lead 206,contacts CC2, and lead 207 to the actuator 67, and breaks theclutch-engaging circuit at CCl.

With both units in operation, the power controls may be moved into thereverse thrust range or the governing range for taxiing, and may beadvanced in thc governing range for takeoff and flight. If it is desiredto declutch one power unit in ight and drive the propeller by theremaining unit only, this may be accomplished by returning the powercontrol on the unit which is to he cut out to Stop to energize theclutch disengagement eircuit from bus 1 through wire 210, contact TSa,and wire 209.

The clutches may not be simultaneously engaged to start both thepropeller and an idle unit from a running unit. With the unit control atStop, the clutch engaging circuit of that unit is broken at T6. lf it ismoved from Stop, the engaging circuit of the other unit clutch is brokenat speed switch S3 of that unit, which is open below 11,000 unit R. P.M. Switch S3 of the second unit is not bypassed until the coolant hasbeen shut off in the first clutch engaged.

The system also provides for starting the power units one after theother from a windmilling propeller in flight, a feature which isprincipally for use in multiengined aircraft. Wtih the power units outof operation, all relays will be deenergized and, ordinarily, thepropeller will be feathered. If feathered, the propeller is unfeatheredand allowed to gain speed. The feather relay when deenergized suppliesbusses 3 and 4. Assuming that the A unit is to be started, the powercontrol lever is moved into the governing range, closing throttle switchT10, establishing a circuit from bus 4 through T10, lead 216, backcontacts CG2 of the ground start relay, lead 193, B unit throttle switchT7', lead 194, and contacts CC1 of the coolant shutoif relay to theengagement line 195, causing engagement of the clutch in the mannerpreviously described. The ignition and fuel circuits operate aspreviously described, and the A unit is thus brought into operation. Thecoolant control relay CC is energized to shut off the coolant when theunit reaches l`l,000 R. P. M. under control of speed switch S3 in acircuit from bus 4 through back contacts CTZ and CTS. When the coolantis shut off, the second clutch engagement relay CS of the B unit isenergized as previously described.

If the B unit is started first, as soon as engagement is completed andthe coolant is shut off, the clutch actuator 67 energizes the secondclutch engagement relay CS, contacts CS2 of which bypass throttle switchT7 so that the A unit may be started. The clutch is engaged by a circuitfrom bus 4 through T10, line 216, back contacts CGZ, line 193, contactsCS2, line 194, contacts CC1, and line 195 to the clutch actuator. Thecoolant is cut E when switch S3 energizes relay CC.

This sequence of operations provides an alternative method for startingthe second power unit on the ground. In this method, the power controlof the first unit started is advanced into the governing range. With therst power unit in the governing range, the second power unit may bestarted by advancing its control lever into the governing range, theclutching control sequence corresponding to that just described. Lead319 branching from lead 209 relates to the throttle block circuits, tobe described.

Propeller control circuits The functions of the propeller controlcircuits are, in general, to establish the operating conditions of thepropeller, to coordinate these conditions with the operation of thepower units, and vice versa, and to Aestablish a system of controleliminating conflicts between the `runit controls. The invention is notconcerned with the control elements of the propeller per se except asthey are components of the overall control system. It is contemplatedthat the system may be used with propellers of various types as, forexample, propellers with either hydraulic or electrical pitch control,and with `various control arrangements, as long as the characteristicsof the propeller control system are compatible with the overall controlsystem of the invention. The principles of the invention also lendthemselves to control with loads other than propellers. For this reason,and in the interest of conciseness, we will not concern ourselves withthe details of the propeller governor .and controls, and the generalnature of these controls will be discussed only sufficiently to explaintheir relation to the power plant control which is the subject of thisinvention.

In general, to carry out the purposes and advantages of the invention asembodied in an aircraft power plant, the propeller should have threephases of operation: lt should be capable of being feathered, that is,brought to a blade angle of approximately 90 degrees so that relativewind provides no substantial turning moment and the drag ofthestationary propeller is a minimum.

Secondly, the propeller should have a variable pitch range for normalpropulsive operation in which the pitch of the propeller is regulated bya speed governor. in this type of control, with the speed set ata'desired value, the pitch of the propeller will depend principally uponthe power input to 'the propeller, the air speed, and air density, inaddition, of course, to the constants of the installation which dependupon the physical' form and dimensions of the propeller. -v

Thirdly, the propeller should have a range of 4eor'itrol in which thespeed governor is inoperative and the `'blade angle lis controlleddirectly through a range of 'positive and negative values. This-controlis desirable principally for ground operation. This range also Vprovides`for Asetting the propeller to a blade angle in which the propellertorque requirement is a minimum. Propellers of the characteristicsstated are known yand are available for aircraft installations.

A Vpropeller having the three modes of operation-re-y ferred to aboveand `capable of external rcontrol to pro-` vide these modes of operationis shown in Blanchard-et al. Patent 2,307,102. The preferred propellerforuse finrconnection with this invention is a development lof VtheBlanchard et al. patent propeller, generally as 'shown in Dinsmore etal. Patent 2,669,312, which discloses a propeller, including anelectrical actuator yand an electronic governor, of the typepreferablyemployed with this invention.

The control Asystem of the invention includes an arrangement by which noconflict arises when the power control levers of `the units are atdiiferent positions, either through misalignment or because one unit isoperating at reduced power or is cut out. The system 'further includesinterlocks between the power control levers which, however, will bediscussed in connection with Figure `9. The coordination of the powerunit control with the propeller depends in large measure upon'coordination of the slots in the controlling cams 111, 112,

and 113 (Figure 2), and also upon certain interlocks yto feather thepropeller, also controls the -energization of busses 3 and 4 yfrom whichthe starting, ignition, fand clutch circuits are energized, aspreviously described;

and, in addition, the feather relay controls `the energization of thefuel shutoff valve solenoid and certain `of the Aclutch circuits bymeans of contacts 'of the relay.

The Apreferred embodiment of this 4portion of the system is illustratedin Figure l8. The pilots feather switch 74 controls feathering andunfe'athering-of thepropeller, energizing the propeller actuator 76vthrough appropriate circuits. The actuator is coupled to limit fswitches'76f, 76g, 76p, and'76n :which yprovide limits for-its operation over4the various vportions `of the propeller `actuating range. p 'Theseswitches are closed :over the'ranges indicated 'by the bars in Figure-12. AIn general, apart yfrom the feather switch, actuator 76 iscontrolled by potentiometers 127 and 128 in the unit controls S3-and 53'which transmit to the actuator'signals establishing'blade angle orpropeller speed governor setting. The actuator is controlled by thesepotentiometersthroug'h a discriminator and follow-up circuit to bedescribed.

The actuator also transmits electrical speedtsignalsto the electronicygovernor 81 'for the propeller which reeeives an input from `thepropeller-driven alternator 84 ysignalling actual propeller* Shaftspeed. The propeller governor may be of the type disclosed in U. S. TPatent 2,669,312. The propeller governor actuates hydraulic valvecontrolling solenoids HD andfHl iin-the propeller which act to 'increaseand decrease the pitch, respectively, when the propeller is undergovernorcontrol. The Ypropeller control solenoids vI-ID 4.and 'Hl andthe valve operate'd thereby may be of the typetdescribed in U. LS.'Patent No. 2,630,136, 'Brandes et al. (filed Tune 8, Y19219). Normally,the governor controls the solenoids'thro'ugh lines 283 `and 284. Powerfor operating a pump 'toivary the propeller pitch is taken from thepropeller shaft.

For feathering and unfeathering, an electric motor 300 drivesa pump,which is thus operative when the propeller is not rotating. Motor 300 isenergized from the aircraft A. C. power system, represented by athreephase generator 301, under ,control of a transfer relay PI and acentrifugal switch S5, which is closed on the ,motor contacts below alow propeller speed. Switch S also breaks the circiut to solenoids HDand HI below the operating point of the switch. A governor disconnectrrelay PL also is provided to cut the circuits from the governor tocoils HD and Hl. The manner in which relays PJ and PL are energized willbe made clear.

; We may start by tracing the circuits by which the propeller isunfeathered, since this is a prerequisite to bringing the power plantinto operation. First, the power Control lever of one power unit must beadvanced into Vthe governing range to close throttle switch T11 or T11.

Then, by pushing feather switch 74 (that is, moving it to lthe left inFigure 8) yand holding the switch in this position, a circuit isestablished from the nacelle bus 151 through the feather switch, line231, throttle switch T11 or T11', and line 232 to energize the unfeatherrelay PE. The pilot switch 74 also energizes transfer relay PJ throughline 233, connecting the feathering pump tuator 76 is energized from theunit controls 53 and 53.

Considering the unit control 53, for example (the two being identical),cam follower 124 (see also Figure 2) adjusts the movable contacts of twopotentiometers 127 and 128. Potentiometer 127 is connected in a seriescircuit from 'bus 2 through a fixed resistance 246 and potentiometerresistor 127 to ground. Potentiometer 128 is connected between groundand bus 2 through a variable adjusting resistance 247. The movablecontact 128a of potentiometer 12S transmits a position signal for theactuator 76. The corresponding potentiometer 128 of the B unit controltransmits a signal in the same manner. Potentiometers 127 and 127',which are varied concurrently with potentiometers 128 and 128',respectively, transmit signals to a discriminator relay PR in thecoordinating control 72 which selects for transmission to theactuatormotor the signal from potentiometer 128 or 128 which is of greatermagnitude, thereby preventing conicts and facilitating the cutting outof one power unit. The discriminator relay PR, which is of a polarizedtype, comprises coils PRl and PR2 connected in series between thecontacts of potentiometers 127 and 127. As will be apparent, if thecontact 127a is farther advanced from ground than Contact 127e', currentwill v ow from contact 127a `to contact 127a', and vice versa.

When current thus ows from contact 127a, the relay contact is closed onxed contact PRa which is connected to potentiometer contact 128:1. Thecircuit from Contact 128:1 to contact PRa passes through throttle switchT12, which is closed on contact T1251 except in the Start position. Thediscriminator relay movable con-- tact is biased to remain closed on oneor the other of contacts PRa and PRb, and will not remain open. Themovable contact of relay PR is connected through line 248 to a follow-upcontrol relay PF for the actuator motor 76.

Since the propeller has been in feather or maximum pitch, the actuatoris at the maximum pitch point of its range of movement. When the powerlever 49 of the A unit is advanced, an electrical signal is transmittedby contact 128a through switch T12 and contacts PRa to line 248 whichcalls for a movement of the actuator toward decreased pitch. Thefollow-up system by which the motor 76 is controlled includes apotentiometer 249 ture 76a of the actuator motor. Contact 249a isconnected through lead 251 and the coils of the polarized follow-uprelay PF to signal line 248. The potential tapped 0E by contact 249:1 isthus balanced against the potential of contact 128a or 12811. When thepotential at 128a is higher, current ilows from line 24S to line 251,engaging the movable contact of relay PF with the increase pitch contactPFi. If the current flows through the relay PF in the oppositedirection, the movable contact engages the decrease pitch contact PFd.Relay PF is biased to open position, and leaves the motor-energizingcircuits open unless the relay coils are energized. These contactsenergize circuits to operate the armature 76a in the appropriatedirection to match the signal transmitted by the unit control. When theposition of the actuator corresponds to that transmitted, the potentialsat 249e and 123a balance, and the motor circuit is opened at the movablecontact of follow-up relay PF.

The motor energizing circuit to unfeather may be traced from bus 2through line 252, contact PFd, line 253, limit switches 76n and 76p inseries, line 254, the contacts of unfeather relay PE, line 256, backcontacts P11 of the increase pitch relay PI, the coil of the decreasepitch relay PD, and line 257 to ground. Relays PI and rPD are referredto as increase pitch and decrease pitch relays in terms of theirfunction in blade angle control. The front contacts of relay PD close acircuit from bus 2 through line 25S, front contacts of relay PD, line259, armature 76a and brake release coil 761' of the actuator motor,line 261, back contacts P12 of the increase pitch relay, and line 257 toground. Coil 76r, which is energized with the armature, releases anormally engaged brake which holds the motor shaft. The eld 76s of theactuator motor is energized directly from bus 2 through line 258. Whencurrent ows as just described from line 259 through the armature to line261, the motor is rotated in a direction to operate the actuator todecrease the pitch of the propeller. The exact position at which themotor stops will depend upon the position of thc power control lever,but is immaterial. The motor will bring the propeller actuator into thegoverning range, and therefore the propeller out of the featheredposition into the range of pitch angles in which the governing controlis effective, by a mechanical signal from the actuator to valves in thepropeller which control the unfeathering A' operation, fluid beingsupplied to effect the operation by the feathering pump motor 300. Thefunction of relay PE is to bypass contacts PH1 of the feather limitrelay which are open when the propeller is completely feathered. Thisarrangement prevents unfeathering unless the switch 74 is closed.

The feather limit relay PH is energized from bus 2 through line 291 andlimit switch 76j when the actuator is at the feather limit position(Figure 12). When the actuator leaves this position, relay PH isdeenergized at switch 761. Back contacts PHl shunt the contacts ci'unfeather relay PE and maintain the decrease circuit to the motorenergized.

It will be further understood that, by appropriate movement of thecontact 12ga, for example, the propeller may be taken out of thegoverning range into the blade angle control range in which blade anglespreferably from about plus twelve degrees (just below the minimum pitchin the governing range) to about minus twenty degrees may be set by thepower control and actuator without reference to propeller speed. Themaximum negative pitch is obtained by movement of the control lever tothe two degree position in the reverse thrust slot. As the lever ismoved forward in this slot, the pitch increases plus twelve degrees justbelow Operational Idle. At and above Operational Idle the propeller isin the governing range in which the pitch may vary, depending upon theoperation of the governor, but is always greater than twelve degrees. Inthe governing range, the actuator operates through speed control ratherthan direct blade angle control. The feather position is beyond the gov-.erning range and is obtained by action of the actuator when the unit isstarted. This control is elected by the throttle switches T12 and T12',which are closed in he FStart position of the power control on contactsT1-2b and T125', respectively. As previously `pointed out, switches T12,unlike the other throttle switches which respond only to the angularposition of the shaft 108 (Figure 2)., 'are actuated only when thecontrol lever is in slot 101 and remain closed on contact Tl2a in thereverse thrust range. Thus, when the A unit control is in Start, contactPRa of the discriminator relay is connected through contact T12band line263 with an adjustable contact point of voltage dividing resistor 264.Resistor 264 is connected in a circuit from bus 2 through fixed resistor266, tapped resistor 264, and yfixed resistor 267. By adjusting thevariable contact of the voltage divider 264, it may be set up totransmit a potential signal to the actuator which will bringthepropeller exactly to the minimum torque posion. The B unit control 53is identical in this respect with the A unit control, so that movementof either unit to the Start position with the other unit in Stop'willenergize the discriminator relay PR and shift the throttle switch T12 totransmit an exact minimum torque signal to the follow-up relay PF.

Actuator limit switch 76p opens 'at the actuator position correspondingto minimum governed speed, just before the actuator enters the positioncorresponding to maximum positive pitch in blade angle control. However,throttle switches T13 and T13 are closed inthe blade angle control range(below Operational Idle and above- Stop), and bypass switch 76p. Thus,When the control is shifted to Start, and the signal calling fordecrease of blade angle to zero closes contacts PFd, the actuator motoris energized as previously described except that the circuit ismaintained through switch T13 a'fter limit switch 76p opens. Theactuator motor thus drives until the follow-up signal from potentiometer249 opens the circuit at contact PFd with the propeller in startingcondition.

A quadrant lock prevents the actuator from leaving rthe governing rangeat either end unless `the lock is released by energizing a solenoid QL.When the power control levers are below Operational Idle and thepropeller is unfeathered, a circuit is 'completed from bus 2 throughline 2811, throttle switches T14 and T15l or T14 and T15., line 282, andback contacts 15H3 o'f the `feather limit relay to energize a quadrantlock release solenoid QL. Switches T14 are clo'sed in the blade anglecontrol frange-and switches Tre' are rclosed in this range and Stop.Thus, this 'circuit will be completed at the throttle 'switches vif bothpower controls are in the blade angle control range, 'o'r 'if 'either'is in this lrange and 'the other y :is at Stop.

"Whenthe quadrant lock is released, the governor circuit tothep'ropeller through lines 283 and 284 is also broken'at relay PL,which is energized 'from bu's 2 through throttle `switches T114 and T15and line 282. i Thus, when the power control is moved from the gove'rnngrange 'intoe'ither the 'reverse `thrust or the starting range, thequadrant lock is released and 'the -governor cir- *cu'it to 'coils HDand Hl is opened.

"When the propeller has been set at minimum torque blade angle, b'o'thpower units maybe lb'rough'tinto operafti'on, a's previously described.Assuming that the units have 'been started andorre 'or both powercontrols is moved To 'Operational Idle, which is the lowest 'point ofthegoverning range, "o'r above, a signal 'is transmitted to the acltuator76 to drive into the governing range. The signal calling -for 4rotationyin the increase pitch direction operates Y Ythe relay 'PFto complete acircuit "from'bus`2 4through line tion.

tional position of the actuator motor may control av 22 252, contact PFi, line 271, contact 76g, line 272, contacts PH2 of the feather limitrelay, line 273, coil PI ofthe increase pitch relay, and line 257toground. Relay PI completes a circuit from bus 2 through line 258,front contacts P12, line 261, coil 76r, armature 76a,.line 259, .backcontacts of relay PD, and line 257 to ground.' This supplies currentthrough the armature in the reverse direct to that previously described,driving the actuator in the increase pitch and increase speed direction.The armature 76a stops when the follow-up relay is balanced.

When either power control is moved into the governing range, thegovernor disconnect relay PL and quadrant lock release solenoid QL aredeenergized by throttle switches T14 and T15 or T14 and T15.

As the actuator enters the governing range, it ceases'to set propellerblade angle and 'operates by way of .the governor 81 to which ittransmits a'propeller speed signal. `It` the power control is advancedfartherto call for higher power output and higher propeller speed, theactuator motor 76 will follow the signal up to the maximum propelletspeed setting, at which point limit switch'76g opens.

The operation of the `'propeller governing system may now be outlined,so far as it relates to the present inven- Although many arrangements bywhich the rotapropeller governor, and many Ways by which propeller speedmay be transmitted tothe governor, are available, a suitable one isillustrated diagrammatically The actuator motor 76, through a mechanicalconnection `schematically indicated at 275, moves the contact point ofthe potentiometer 27 6 energized from the electronic governor 81,supplied by bus 155. The potentiometer tap takes oft a fraction of thepotential across the potentiometer and feeds this potential into thevelectronic governor through line 277. The ratio of 'the potential online 277 to that across the resistor 276 constitutes a desired speedsignal input to the governor 81. The alternator 84, which is driven bythe propeller shaft, `feeds an actual speed signal into the governor.By'mechanismsin the governor, which are immaterial 'to the presentinvention, the desired speed and actual speed signals are compared andany discrepancy affects the output signal of the governor, which istransmitted through lines 283 and 284, the contacts Ior relay PJ, andswitch S5 tothe solenoidcoils HI and HD in the propeller. These coilsactuate a hydraulic valve which, by mechanismrimmat'efria'l to theinvention, controls the transmission of 'uid from a pump in thepropeller, which is preferably driven by the vpropeller shaft, tohydraulic motors which vary the pitch of the propeller. If the propellerspeed exceeds that vsignalled for, the coil HI acts to increase thepropeller jpi'tch and thus its resistance to rotationuntil the speed .isreduced to the desired value. Tf 'the speed is too low, coil HDdecreases the pitch so that the speed increases. As will be apparent,the coils vHD and HI could be field coils of an electric motor to adjustthe propeller pitch, or could be relay coils controlling anelectricalsys'tem. They may be energized alternately during a cycle,with the resultant eiect depending upon 'the relative time ofenergization of the two solenoids during the cycle. The structure of thepropeller and the ineans by `which the governor regulates the vpropellerfare immaterial to the invention except that thecharacteristi'cs ofthepropeller and its governor must 'accommodate themselves to the control'system of the invention. The 'speed selected for the propeller iscoordinated with thepower lcontrol `of `the units by cams V111 and 1134in the unit controls, .as previously described in connection withyFigure 13.

We may now consider the circuits involved in blade angle control ofthepropeller in the reverse thrustrrange employed for ground operations.With the Vgovernor disabled and the quadrant block released `bymovementof the power control levers into the reverse `thrust .slot 104,the potentiometer contact 128a or 128a' transmits a signal to theactuator to drive into the blade angle control range and .setpropellerpitch, which is .accomplished -in

